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In the Washington Post article Catherine Rampell raises an important point with regard to the emerging debate over whether to tax soda.

Instead of arbitrarily singling out one category of bad foodstuff for taxation — and the categories of bad foodstuffs will always be somewhat arbitrary — a more effective route to reducing consumption of excessive sugar or calories might be a universal, graduated sugar or calorie tax.

But even that still doesn’t quite seem fair or, for that matter, efficient. After all, a calorie tax would also hit people who consume more calories because they are very active, such as marathoners. Besides being regressive, a tax on calories or sugar would also effectively, if unintentionally, make it more expensive for trim people to exercise.

In other words, a lot of inputs go into determining whether a person is obese. Taxing some of those inputs distorts the relative prices of those inputs, but it doesn’t necessarily change the desired output: obesity rates.

Which raises the question: Why not just target the output, rather than some random subset of inputs? We could tax obesity if we wanted to. Or if we want to seem less punitive, we could award tax credits to obese people who lose weight. A tax directly pegged to reduced obesity would certainly be a much more efficient way to achieve the stated policy goal of reducing obesity.

Yet, people don't seem to like the idea of a fat-person tax. Why not?

Maybe it’s because they’re regressive (but so are soda taxes). Maybe it’s because it sounds like we’re shaming fat people (but arguably so does any policy aimed at reducing obesity). Maybe it just feels unfair to tax people based in any way on their genes, which, like diet and exercise, can also be a determinant of weight.

But if we assume it’s impossible for obese people to lose weight by any combination of inputs they do have control over, it’s hard to simultaneously argue that making one of those inputs more expensive could lead to some nationwide weight-loss miracle. Pop goes the pop-tax rationale.